It's taken nearly 200 years, but scientists in Arizona and Europe have finally teased out how the molecular switch for sex gradually and adaptively evolved in the honey bee. The first genetic mechanism for sex determination was proposed in the mid-1800s by a Silesian monk named Johann Dzierson, according to the study's co-author and Arizona State University (ASU) Provost Robert E. Page Jr. Dzierson was trying to understand how males and females were produced in honey bee colonies. He knew that the difference between queen and worker bees – both females – emerged from the different quality and quantity of food. But, what about the males, he asked. Dzierson posited that males were haploid – possessing one set of chromosomes, which was confirmed in the 1900s with the advent of the microscope. Under the magnifying lens, researchers could see that eggs that gave rise to drones were not penetrated by sperm. However, how this system of haplodiploid sex determination ultimately evolved at a molecular level has remained one of the most important questions in developmental genetics. In the December 5, 2013 issue of Current Biology, Dr. Page and Dr. Martin Beye, lead author and professor with the Institute of Evolutionary Genetics in the University of Duesseldorf, Germany, and their collaborators laid out the final pieces of how these systems evolved in their article "Gradual molecular evolution of a sex determination switch in honey bees through incomplete penetrance of femaleness." The authors studied 14 natural sequence variants of the complementary sex determining switch (csd gene), for 76 genotypes of honey bees. While complex, the researchers had several tools at hand that their predecessors lacked to solve this sexual determination puzzle.